1. Field of the Invention
The present invention relates to the tempering of flat glass sheets and particularly, glass sheets that are tempered while conveyed first over a gas hearth bed in the form of an apertured roof of refractory material for a hot gas plenum chamber of the type depicted in U.S. Pat. No. 3,338,697 to McMaster et al. followed immediately by conveyance through opposed blasts of chilling medium. Such gas hearth beds have gas supply passes interspersed with exhaust passages to provide a thin bed of gas to support the glass sheets during their heating that procedes rapid chilling. The arrangement of supply and exhaust passages is such to minimize any gross variation from the support pressure provided throughout the extent of the gas bed.
The present invention particularly relates to the tempering of flat glass sheets that are supported horizontally or substantially horizontally during their thermal treatment while supported successively on hot gas and cool gas. Glass sheets so treated tend to develop an upward kink or curl, particularly at their leading edge. This upward curl is noticeable even when associated with relatively small glass sheets and may be located anywhere along the length of the sheet, most frequently in its leading edge portion. However, when tempering glass sheets having one or more dimensions greater than one meter, the leading edge of the glass sheet is more likely to develop an enlarged, upward curl. In such larger glass sheets and in sheets of smaller dimension heated on a gas hearth, the leading edge curl is sometimes sufficiently severe to cause the glass sheet to depart from the flat shape desired by an amount sufficient for the glass to be outside tolerance requirements. Such severe edge curl leads to customer rejects. Leading edge curl is the deviation from flatness of the glass sheet measured in the direction of its thickness in its initial one foot (30 centimeters) of length. Sometimes, maximum curl is observed away from the leading edge. In such case, maximum curl is the term used to identify the maximum deviation from flatness along any one foot of a glass sheet dimension.
Another defect of tempered glass is known as warp. Warp is the term used to define the maximum deviation from flatness along an entire dimension of a glass sheet. In rectangular glass sheets, warp is measured along the leading, trailing and side edges.
In well-known commercial procedures for tempering glass sheets, the sheets are first heated to a temperature corresponding substantially to the softening point of the glass and then are chilled rapidly to a temperature below the annealing range of the glass to impart a temper. Conventionally, the sheet is heated while transported through an enclosed hot atmosphere of a furnace and propelled forwardly through the furnace exit into a cooling station comprising upper and lower plenums which flush the opposite surfaces of the sheet with streams of suitable tempering medium, such as air under predetermined pressure applied through arrays of nozzles which chill the glass and impart the desired stress patterns therein. Cooling air is directed against the opposite surfaces of the sheet while the sheet moves between the upper and lower plenums in order to impart the temper desired.
It is also well known to arrange the size and distribution of exhaust passages relative to the supply passages to provide a pressurized gas bed whose pressure does not vary greatly between different regions of the gas supply bed. Prior to the present invention no effect was made to provide exhaust passages in the exit portion of the gas hearth bed due to the difficulty of drilling exhaust passages through the end wall structure at the end of the gas supply bed. Consequently, the exit portion of the gas supply bed was provided with smaller diameter gas supply passages than those provided throughout the rest of the bed. Nevertheless, warp and kink developed in glass sheets tempered on apparatus employing gas hearths so constructed.
2. Description of the Prior Art
U.S. Pat. No. 3,223,501 to Fredley et al. discloses a gas support bed or hearth forming the roof of a plenum chamber where exhaust passages are interspersed among gas supply modules to insure a relatively constant gross pressure profile throughout the extent of said gas hearth. The modules of the bed are of discrete, metal construction and thermal expansion of individual modules at different rates warped the upper surface of the gas bed.
U.S. Pat. No. 3,338,697 to McMaster et al. discloses a slightly different arrangement of supply passages and exhaust passages in a gas hearth composed of a low expansion refractory material such as quartz. The low expansion quartz has less tendency to warp than the metal module bed of the Fredley et al patent. However, quartz is brittle so that it is not advisable to fabricate separate modules of quartz with spaces between the separate modules as is possible with metal modules. Furthermore, at the downstream end of the gas hearth, it is virtually impossible to fabricate exhaust holes through the downstream end portion of the plenum chamber roof that forms the gas hearth bed and through the end wall that supports the downstream end of the roof, particularly in a gas hearth bed that is already in existence.
U.S. Pat. No. 3,923,488 to Pytel et al. discloses a technique for reducing upward curl at the leading edge of a glass sheet conveyed through a heated area and a cooling area by applying a cooling medium toward a portion of the lower surface of the glass sheet only in spaced relation to its leading edge. This application is started after the glass sheet leaves the furnace and before the glass portion reaches a position where both its upper and lower surfaces are cooled by exposure to opposing blasts of tempering medium. The entire length of glass sheet except for the leading one foot or so of length is subjected to this differential cooling treatment.
It would be desirable to develop a different method for controlling warping, kinking or curling that does not depend on controlling the timing of means to apply tempering medium differentially to selective portions of glass sheets.